Titan hangs on to its secrets

Cassini's close fly-by of Titan, Saturn's largest moon, has left scientists with no clear idea of what to expect when the Huygens probe lands on the alien world, despite the amazingly detailed images they now have of the surface.

Pictures from Cassini's fly-by revealed clearly delineated areas of dark and light on the moon's surface; but interpreting the features is a challenge. Scientists at the European Space Agency (ESA) think the lighter areas indicate a rough surface, and the dark areas are smooth. The surface shows few signs of impacts, indicating that it is geologically young. The Huygens team, headed by Professor John Zarnecki of the Open University, says that further analysis of the visual and radar data should help clarify things.

Data from Cassini's electron spectrometer contain clues to the composition of Titan's upper atmosphere. Dr Andrew Coates of the Mullard Space Science Laboratory in London told the BBC that the data contains the "tell-tale finger prints of photoelectrons and Auger electrons" needed for this analysis.

On Christmas day Cassini will despatch the Huygens probe, which will begin its descent through the atmosphere on 14 January 2005. The ESA says the streak of light created when the craft heats up in the atmosphere might even be visible from Earth.

The odds are against spotting the probe as it streaks across the sky of Titan. But we are in with a chance: according to the ESA, "the best location to be looking from happens to coincide with the largest single telescope in the world: the 10-metre Keck telescope". Keck sits on the summit of Mauna Kea, a dormant volcano on Hawaii, and will be perfectly aligned with Titan when the probe hits the atmosphere. ®